The moon, our closest neighbor, remains in many ways a mystery to planetary scientists—a destination tantalizingly close, yet frustratingly difficult to reach. But even though an Earthling has not set foot on the lunar surface since Apollo 17 in 1972, samples brought back during that mission continue to grace the science community with insights into the nature of Earth's satellite.

A new study tracing the history of one of those moon rocks, published in this week's Science, adds fuel to a long-running debate over the source of the faint magnetism present on the lunar surface. (The moon, unlike Earth, does not have a dynamo within—a churning molten iron core that produces a magnetic field.) Some researchers believe that bombardment by meteoroids and asteroids might account for the remnant magnetism, but the Science study supports the theory that the moon once had its own magnetic field and, therefore, its own core dynamo.

Refining theories of lunar geology may help to shed light on the moon's origins and evolution. Lead study author Ian Garrick-Bethell, then a graduate student in the Massachusetts Institute of Technology's Department of Earth, Atmospheric and Planetary Sciences, says that the dynamo theory, if correct, would provide great insight into the moon's interior.

"If you could infer the moon had a liquid core in the past, you could infer that it still has a core today—a core doesn't really go anywhere," he says. "You might not be able to constrain whether it's liquid or solid, but there are other studies that suggest that the moon presently may have a liquid core." This core, if it exists today, would not be large or active enough to sustain a dynamo.

The paper's authors examined a moon rock, known as troctolite 76535, believed to be "unshocked" or not significantly altered by the meteoric impacts that have ravaged the lunar surface since its origin. (Troctolite 76535 is the oldest such unshocked sample, making it a unique record of lunar history.) By tracing its thermal and magnetic history with a suite of measurements, Garrick-Bethell and his colleagues determined that the rock was last remagnetized 4.2 billion years ago. (The moon itself is thought to be some 4.5 billion years old.)

The magnetization appears to have been encoded in the rock during a period of cooling thousands of years long. That cooling time frame would not have allowed for meteoric impact magnetization, which usually fades within a day or so, to be locked in, Garrick-Bethell says, pointing to a more permanent internal magnetic field on the moon.

"We know that around 4.2 [billion years ago] there was a long-lived field [that lasted] at least several million years," Garrick-Bethell says. That timing, says planetary scientist Dave Stegman, a research fellow at the University of Melbourne in Australia, is "actually quite serendipitous—that's exactly the time in lunar history during which a dynamo could either be dying down or just starting to ramp up."

Stegman supports the lunar dynamo theory, having published a 2003 study on it Nature, but cautions that the new results are based on one data point—in this case, one lunar sample. He says more samples are needed to fine-tune the timeline.

"We are very much limited by the amount of material," Garrick-Bethell acknowledges, but notes that researchers have devised many creative ways to extract information from a limited sample pool. "We really try to do as much as we can with as little as we can."

With more manned lunar exploration a decade or two away, the mysteries of the moon's geologic past remain in the realm of competing hypotheses. But pinning down the origin of lunar magnetism brings scientists closer to unveiling the rest of its evolutionary secrets. "One thing is for certain," Stegman says. "The existence or nonexistence of the lunar dynamo has huge implications on how we understand the evolution of the moon."

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